Fertilizer material from apatite

ABSTRACT

Apatite is conventionally made into a fertilizer material by treatment with a strong acid, requiring capital-intensive industry. Hydroxyapatite is treated above 1100 C. with alkali but world reserves are problematic. The invention treats apatite at 900 C. with sodium aluminosilicate/carbonate and siliceous material in quantities to keep the composition in terms of CaO, SiO 2 , Na 2  O and P 2  O 5  in or near the ternary system Ca 2  SiO 4  --Ca 3  (PO 4 ) 2  --CaNaPO 4 .

This invention relates to making a fertilizer material from apatite.Apatite is an insoluble phosphorus-containing mineral, approximating toCa₅ (PO₄)₃ (F,OH,CI,1/2CO₃), and the phosphate content must be renderedsoluble for fertilizer use.

Apatite, the most abundant terrestial form of phosphorus, isconventionally treated with a strong acid such as nitric or sulphuricacid to render the phosphate soluble. This requires a capital-intensiveindustry.

Another known way of treating apatite is the `Rhenania process`described in British Patent Specification 301022. The apatite is mixedwith Na₂ CO₃ to give a molar ratio of Na₂ CO₃ /P₂ O₅ ≃1.0, while at thesame time sufficient SiO₂ is also added to combine with excess CaO. Thereactants are ground together and calcined in a rotary kiln at 1100C.-1200 C. for approximately 2 hours. Fluorine is said to be retained inthe process, although steam is sometimes admitted to the kiln before1000 C. is reached in an attempt to remove at least part of thefluorine. The sintered product may be used directly as a slow-releasesource of P or it may be subsequently extracted with hot aqueous Na₂ CO₃solution, giving either Na₃ PO₄ or Ca₃ (PO₄)₂. This process requireshigh temperatures, and hydroxy-rich apatite, which is indigenous tocentral Europe and occurs in a few other regions of the world, some ofwhich are of questionable reliability.

Hard mineral apatite (richer in chlorine/fluorine) is widely availablein Sri Lanka, India and East Africa and is often a by-product from othermining operations. The present invention seeks to make fertilizermaterial from such apatite at a lower temperature than the 1100 C.-1200C. required in the Rhenania process and without using acid. Sri Lanka isunderstood to have an indigenous alkali (NaOH) industry. (While it doesalso produce hydrochloric acid, this is not a suitable acid for treatingapatite). The alkali is readily convertible to sodium carbonate. To makefertilizer material from apatite according to the present inventionrequires siliceous material, and this is also widely available asquartz, sand or potash felspar (an alkali metal aluminosilicate).Addition of the last-named in small proportions also has the advantageof introducing available K₂ O, and the same might be said of mica.

Accordingly, the present invention is a method of making a fertilizermaterial from apatite, by roasting apatite at up to 1100 C. with acarbonate and/or aluminosilicate of an alkali metal in an amount suchthat the molar ratio apatite (as P₂ O₅): alkali metal is 1: at least 3and in the presence of sufficient siliceous material to keep thefree-lime content of the fertilizer material below 2 weight % and toinhibit formation of tetracalcium phosphate.

The molar ratio apatite:alkali metal is preferably from 1:3 to 1:10,more preferably 1:3 to 1:5, for example 1:4.

The molar ratio apatite:siliceous material (as SiO₂) is preferably from1:0.75 to 1:1.0.

The roasting temperature is preferably below 1000 C., and desirably atleast 800 C., more preferably at least 850 C., most preferably from 880C., to 950 C., for example 900 C. The duration of roasting need notexceed 2 hours, and is preferably at least 1 hour.

Preferably the apatite, the siliceous material and the carbonate and/oraluminosilicate are pressed together (e.g. pelletised) before theroasting. This appears to enhance the rate of reaction.

The invention extends to the fertilizer material made as set forthabove, optionally admixed with other agriculturally acceptablecomponents.

The reasons for avoiding excessive free lime and tetracalcium phosphate(i.e. why siliceous material is added) are as follows: Free lime iscapable of causing skin burns, reacts with moisture thereby causingcaking and may make the fertilizer, and hence the soil, too alkaline.The phosphate in tetracalcium phosphate Ca₄ P₂ O₉ is all soluble, i.e.it is at first sight an ideal fertilizer material. However, Ca₄ P₂ O₉ isliable to conversion in the presence of water vapour, which is likely ina fossilfuel-fired tunnel kiln, to CaO (or Ca(OH)₂) plus insolublehydroxyapatite, one of the very materials which the present inventionwas devised to solubilise.

In practice, the quantities of Na₂ O/K₂ O and SiO₂ (which must be addedin order to eliminate the above undesirable phases) are desirably theminimum, as an excess would result in too much dilution of the phosphatephases. A way of determining these is to consider the CaO- and P₂ O₅-rich regions of the system CaO-Na₂ O-P₂ O₅ -SiO₂. The plane ofcompositions lying between Ca₂ SiO₄, Ca₃ (PO₄)₂ and CaNaPO₄ just fulfilsthe condition that CaO and Ca₄ P₂ O₉ should be absent and, furthermore,we find that this plane of compositions constitutes a true ternarysystem at subsolidus temperatures. Its position within the quaternarysystem is shown in the accompanying drawing.

Table 1 records the results of solubility determinations made on puresingle-phase preparations. For present purposes availability is definedby the relation: ##EQU1## The experimental method for determining the2%-citric-acid-soluble P₂ O₅ is given in the Appendix. Ground mineralapatite is poorly soluble: typically only 17-18% of its P₂ O₅ content is`available`.

Amongst the phases having 100% available P₂ O₅ are Ca₄ P₂ O₉,nagelschmidtite, silicocarnotite, rhenanite (a range of solid solutionsaround CaNaPO₄), and an α Ca₂ SiO₄ solid solution containing typically30 wt % Ca₃ (PO₄)₂. The presence of silica in solid solution inrhenanite appears to activate the dissolution of phosphate. Moreover,phase A is not completely soluble unless it too contains silica in solidsolution; Phase A is explained in the footnote to Table 3. Both α and βCa₃ (PO₄)₂ give less than 100% available P₂ O₅. Moreover, while Ca₄ P₂O₉ has 100% availability, it is (as already mentioned) readily convertedto hydroxyapatite by annealing in air, whereby the available P₂ O₅ fallsto 20%. Attempts to form a solid solution, substituting two Na⁺ ions forCa⁺⁺ ions, in the hope that Ca_(4-x) Na_(2x) P₂ O₉ would be lessreactive to water vapour than the Ca₄ P₂ O₉, were unavailing.

CaNa₆ P₂ O₉ was found to be 100% extractable, but is hygroscopic andtherefore undesirable.

                  TABLE 1                                                         ______________________________________                                        RESULTS OF SOLUBILITY STUDIES                                                                    P.sub.2 O.sub.5                                                               SOLUBLE   SOLUBILITY                                                          IN 2%     AS PERCENT                                                          CITRIC    OF TOTAL                                         SAMPLE             ACID      P.sub.2 O.sub.5                                  ______________________________________                                        β Ca.sub.3 (PO.sub.4).sub.2                                                                 34.0      74                                               α Ca.sub.3 (PO.sub.4).sub.2                                                                36.6      80                                               Ca.sub.4 P.sub.2 O.sub.9                                                                         38.9      100                                              Ca.sub.4 P.sub.2 O.sub.9 heated at 1000° C. in air                                         7.8      20                                               Ca.sub.5 (SiO.sub.4) (PO.sub.4).sub.2                                                            30.1      100                                              Ca.sub.7 (SiO.sub.4).sub.2 (PO.sub.4).sub.2                                                      21.9      100                                              α-Ca.sub.2 SiO.sub.4 ss containing Ca.sub.3 (PO.sub.4).sub.2            [Composition = 70.0 wt % Ca.sub.2 SiO.sub.4 ]                                                    14.0      100                                              Phase A - Ca.sub.5 Na.sub.2 (PO.sub.4).sub.4                                                     40.1      90                                               Phase A ss containing SiO.sub.2                                               [Composition = 20 wt % Ca.sub.2 SiO.sub.4,                                    40% Ca.sub.3 (PO.sub.4).sub.2, 40% CaNaPO.sub.4 ]                                                36.4      100                                              β CaNaPO.sub.4                                                                              45.0      100                                              α CaNaPO.sub.4 ss containing SiO.sub.2                                  [Composition =  10 wt % Ca.sub.2 SiO.sub.4,                                   10% Ca.sub.3 (PO.sub.4).sub.2, 80% CaNaPO.sub.4 ]                                                40.5      100                                              CaNa.sub.6 P.sub.2 O.sub.9                                                                       36.9      100                                              Sri Lanka Apatite Sample (1)                                                                      6.3      17                                               Sri Lanka Apatite Sample (2)                                                                      6.2      18                                               ______________________________________                                         Note:                                                                         ss = solid solution; Sri Lanka apatite sample (1) is a sample of pure         apatite from the "leached zone" in the deposit at Eppawela, Sri Lanka.        Sample (2) is a commercially beneficiated sample of apatite from Eppawela     Sri Lanka.                                                                    For explanation of Phase A, see footnote to Table 3.                     

As for the influence of halogens, chlorine is almost entirely eliminatedduring the firing of apatite-containing batches, although most of thefluorine is retained. The fluorine is believed to be present in solidsolution in phases which are soluble in citric acid. When these phasesare dissolved in citric acid, fluorine is probably present in thesolution in the form of fluorosilicate complexes.

The invention will now be described by way of example. The accompanyingdrawing shows a corner of the quaternary system CaO-Na₂ O-P₂ O₅ -SiO₂.The plane Ca₃ (PO₄)₂ -Ca₂ SiO₄ -CaNaPO₄ has been marked out.Compositions on this plane contain neither CaO nor Ca₄ P₂ O₉, andaccordingly are desirable.

Mineral apatite (minus 100 mesh BS) was reacted with Na₂ CO₃ and SiO₂(quartz, minus 120 mesh). The apatite was taken from the "leached zone"of the deposit: Table 2 gives a complete analysis typical of theconcentrate as well as partial analysis of the particular batch ofapatite concentrate used in this study. Microscopically, the apatiteoccurs as anhedral grains, most of which are monocrystals.

                  TABLE 2                                                         ______________________________________                                        CHEMICAL COMPOSITION OF SRI LANKA APATITE                                              Sample EP/N/J   Sample used in                                       wt %     from the leached zone.sup.(a)                                                                 the Examples                                         ______________________________________                                        CaO      55.30           --                                                   SrO      1.18            --                                                   MgO      0.01            --                                                   MnO      0.01            --                                                   Fe.sub.2 O.sub.3                                                                       0.08            --                                                   SiO.sub.2                                                                              0.40            --                                                   P.sub.2 O.sub.5                                                                        40.75           38.10                                                F        1.78            1.70                                                 CI       2.29            2.20                                                 ______________________________________                                         .sup.(a) Analysis reported by the Geological Survey Department, Colombo 2     Sri Lanka (1973). The two samples were believed to be essentially             identical.                                                               

Reaction batches were prepared by blending these raw materials andfiring following a heating rate of 5 C./min at a constant temperaturefor 2 hours. Table 3 records the results of the annealing treatments.The phases present were determined by X-ray powder diffraction but thelimit of detection of unreacted apatite was as high as 5%.

In the absence of SiO₂, mixtures of apatite and Na₂ CO₃ react to producelarge amounts of free CaO. Therefore, as free CaO is deemed to be anundesirable constituent, it is essential to add something to combinewith it: SiO₂ fulfils this role.

Much of the reaction is completed swiftly, even though this apatite iscomparatively coarse-grained. As a rough guide, batches having a molarratio of apatite to Na₂ CO₃ from 1:1.5 to 1:2.0 (i.e. apatite:alkalimetal=from 1:3 to 1:4) gave the most rapid reaction at low temperatures.These batch proportions correspond to a weight percentage of Na₂ CO₃between 20 and 27%. Some excess sodium carbonate was tolerable. If theoptimum proportions of all three components are considered, molar ratiosof apatite:Na₂ CO₃ :SiO₂ close to 1:2:1 are favourable for reaction. Ifthe SiO₂ content is reduced slightly below this optimum, for example to1:2:3/4, a high yield of available phosphorus is obtained, but free CaOis also developed. Similarly, reduction of the sodium carbonate contentleads to incomplete reaction and the appearance of unreacted apatitewhich can only be removed by sintering at 1100°-1300° C.

Table 3 shows the various compositions tried. Examples 1 to 3 areaccording to the invention. Examples A to K are not according to theinvention. Example 1 is shown prior to Example K. It will be seen thatfew of Examples A to K gave any significant reaction below 1100° C., andof those which did, either inadequate phosphate was solubilised (as H)or free lime and sometimes a hygroscopic product resulted (as C and D).The phases present are given roughly in the order: most first.

                  TABLE 3                                                         ______________________________________                                        AVAILABLE P.sub.2 O.sub.5 CONTENT OF SINTERS                                  MADE WITH APATITE                                                             ______________________________________                                               COMPOSITION                                                                   Molar ratio  Weight percent                                            Example No                                                                             Apatite:Na.sub.2 CO.sub.3 :SiO.sub.2                                                         Apatite Na.sub.2 CO.sub.3                                                                     SiO.sub.2                             ______________________________________                                        A        1:1:0          82.9    17.1    0.0                                   B        2:3:0          76.3    23.7    0.0                                   C        1:2:0          71.0    29.0    0.0                                   D        1:10:0         32.9    67.1    0.0                                   E        5:2:8          78.6    6.5     14.9                                  F        4:2:3          85.1    8.1     6.8                                   G        10:8:3         82.9    14.2    2.9                                   H        13:10:11       79.4    12.7    7.9                                   J        1:1:1          75.5    15.6    8.9                                   1        4:6:3          71.5    22.2    6.3                                   K        2:3:3          67.3    20.9    11.8                                  2        4:8:3          66.6    27.5    5.9                                   3        1:2:1          65.3    27.0    7.7                                   ______________________________________                                                2%                                                                            CITRIC ACID                                                                   SOLUBILITY                                                                          Wt                                                                            Per-   Percent PHASES PRESENT                                   EX.  TEMP.    cent   out of  (See abbreviations                               NO.  °C.                                                                             P.sub.2 O.sub.5                                                                      Total P.sub.2 O.sub.5                                                                 below)                                           ______________________________________                                        A    1100     20.5   55      β R + CaO + F ap                                 1300     28.5   76      β R + αR + CaO + F ap                 B     900     23.0   67      β R +  αR + CaO + F ap                     1100     27.8   81      β R + αR + CaO + F ap                      1300     29.5   86      β R + αR + CaO + F ap                 C     900     31.4   90      β R + CaNa.sub.6 P.sub.2 O.sub.9 + CaO                                    + tr. F ap                                           1100     35.0   100     β R + CaO                                        1300     34.7   100     β R + CaO                                   D     900     16.6   100      CaO + CaNa.sub.6 P.sub.2 O.sub.9                E    1100     8.0    23       F ap + αR + βR                            1300     14.4   42       F ap + A                                        F    1100     9.8    27       F ap + αR + βR                            1300     15.8   43       F ap + A                                        G    1100     18.1   49      β R + F ap + CaO                                 1300     18.8   51      β R + F ap + CaO                            H     900     10.0   36       F ap + β R                                      1100     16.5   59      β R + F ap                                       1300     20.4   73       A + βC.sub.3 P + F ap                      J    1100     18.8   55      β R + F ap                                       1300     25.1   74       A + βR + F ap                              1     900     21.7   67      β R + αR + F ap                            1100     29.7   91      β R + αR + F ap                            1300     30.0   92      β R + αR + F ap                       K    1100     21.8   70      β R + F ap                                       1300     24.9   80      β R + F ap                                  2     900     27.0   92      αR + βR +                                                           CaO + tr.                                                                     F ap                                                 1100     30.4   100     βR + αR +                                                           CaO                                                  1300     31.1   100     βR + αR + CaO                         3     900     25.5   91      αR + βR + tr. F ap                         1100     28.1   100     αR                                              1300     28.6   100     αR                                         ______________________________________                                         Abbreviations: R = Rhenanite, CaNaPO.sub.4 ; F ap = Fluorapatite, Ca.sub.     (PO.sub.4).sub.3 F (all remaining apatite having this composition); tr. =     trace. α and β are the high and low temperature forms              respectively.                                                                 C.sub.3 P = tricalcium phosphate. Phase A is believed to approximate to       Ca.sub.5 Na.sub.2 (PO.sub.4).sub.4 and is the crystalline phase defined b     Ando and Matsuno (J. Ando and S. Matsuno, Bull. Chem. Soc. Japan 41 (1968     342.) In the present context, Phase A solid solutions may also contain th     silicon.                                                                 

APPENDIX

Determining the 2%-citric-acid-soluble P₂ O₅ in a sample:

The sample is ground to pass a 100 mesh BS sieve. A 1.0 g sample isextracted with 100 ml of 2% citric acid in a mechanical shaker operatingat 260 oscillations per minute for 30 minutes at 18 C. The resultantsolution is filtered under vacuum using a sintered glass crucible(porosity No. 4) and the filtrate P₂ O₅ content is determined by thevanadomolybdate method, in which the following reagents are used:

(i) Ammonium metavanadate solution, prepared by dissolving 1.12 g ofammonium metavanadate in a mixture of 240 ml of concentrated HClO₄ acidand 260 ml of water.

(ii) Ammonium molybdate solution, prepared by dissolving 35 g ofammonium molybdate in 500 ml of water.

(iii) Standard phosphate solution, 0.2 mg/ml P₂ O₅, prepared bydissolving 0.3835 g of dried potassium dihydrogen phosphate in 1 literof water.

Solutions (i) and (ii) are stable and will keep for some months.

To a 2 ml aliquot of sample filtrate are added 10 ml of the vanadatesolution (i) and 10 ml of the molybdate solution (ii) successively,mixing well after the addition of each reagent. The resultant solutionis diluted with water in a 100 ml volumetric flask.

After 30 minutes the absorbance is measured at 460 nm using a Unicam SP600 colorimeter against a reagent blank solution. The standard phosphatesolution (iii) is used in calibration.

We claim:
 1. A method of making a fertilizer material from hard mineralapatite rich in chlorine and/or fluorine, by roasting apatite at fromabout 880° C. to about 900° C. as a final temperature with a carbonateand/or aluminosilicate of an alkali metal in an amount such that themolar ratio apatite (as P₂ O₅):alkali metal is 1: at least 3 and in thepresence of sufficient siliceous material to keep the free-lime contentof the fertilizer material below 2 weight % and to inhibit formation oftetracalcium phosphate.
 2. A method according to claim 1, wherein themolar ratio apatite:alkali metal is from 1:3 to 1:10.
 3. A methodaccording to claim 2, wherein the molar ratio apatite:alkali metal isfrom 1:3 to 1:5.
 4. A method according to claim 1, wherein the molarratio apatite:siliceous material (as SiO₂) is from 1:0.75 to 1:1.0.
 5. Amethod according to claim 1, wherein the duration of the roasting doesnot exceed 2 hours.
 6. A method according to claim 1, wherein theduration of the roasting is at least 1 hour.
 7. A method according toclaim 1, further comprising pressing together the apatite, the siliceousmaterial and the carbonate and/or aluminosilicate before the roasting.